A piezoelectric energy harvesting device (hereafter, referred to as a “PEH device”) generates the largest electric energy at the resonance frequency, with amplification of the displacement, when the frequency of the environmental vibration and the resonance frequency of the PEH device agree with each other.
The voltage generated by the PEH device is outputted in the AC (Alternating Current) type. A rectifier is used to convert the AC voltage into the DC voltage. The rectifier is composed of four or two diodes, in a full-bridge or half-bridge type. A capacitor for reducing ripple of the DC voltage is connected to the rear end of the rectifier.
The DC voltage, is used to charge another supercapacitor or a battery, or activate an IC etc. Electric energy that is obtained from small vibrations generated in the peripheral environment is, however, not sufficient as power for activating an IC because the magnitudes are too small. Therefore, a method of increasing output by optimizing the size or the shape of the PEH device or using a multilayer has been researched.
A piezoelectric ceramic device has a brittle property, such that it is vulnerable to shock etc. and is limited in increasing the size of the device. Using the multilayer is not suitable for manufacturing the PEH device, because the manufacturing processes are not yet established. As another method of increasing the output of the PEH device, there is a method using single crystals having high coupling efficiency and a large piezoelectric constant, which requires very difficult process for manufacture a multilayer.
As a method for solving the problems, a method of improving output, using piezoelectric energy harvesting arrays (hereafter, referred to as ‘PEH arrays’) can be proposed. A method that uses a tip mass at the end of a cantilever after optimizing the shape or the size of a single PEH device itself is generally used to fit the resonance frequency of the PEH device to the frequency of an environmental vibration (for example, 1 to 120 Hz).
Meanwhile, two frequencies exist, when the PEH device is manufactured by a piezoelectric material. One is the resonance frequency in a short-circuit state in which resistance R (connected to PEH device) goes to 0 (hereafter, referred to as ‘sc resonance frequency’) and the impedance is the smallest, and the other one is the resonance frequency in an open circuit state, in which the resistance R goes to ∞ (hereafter, referred to as ‘oc resonance frequency’) and the impedance is the largest. R is a resistor connected to the end of the PEH device. The resonance frequencies are determined by an effective electro-mechanical coupling constant. In general, the piezoelectric ceramic is small for effective electro-mechanical coupling constant and a piezoelectric single crystal has a large value close to 1.0 (efficiency of converting mechanical energy into electric energy=100%).
In order to measure the basic output characteristic of the PEH device, a resistor R is connected to the end of the PEH device, with a low vibration applied, and the current flowing through the connected resistor R is measured, or the voltages at both ends of the resistor are measured. Since the frequency where the maximum output is generated changes between the sc resonance frequency and the oc resonance frequency, in accordance with the value of the resistor R, the resonance frequency of the single PEH device can be easily measured.
However, since the sc resonance frequency and the oc resonance frequency of the PEH array depend on the mechanical state as well as the electric connection states, the electric properties of the PEH array are measured, with the resonance frequency of the single PEH device made fit the frequency of the environmental vibration, after the PEH array is manufactured. In this case, a desired output cannot be achieved due to output saturation, even if the number of PEH devices is increased.